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Autophagy: It’s in Your Blood

2017; Elsevier BV; Volume: 40; Issue: 6 Linguagem: Inglês

10.1016/j.devcel.2017.03.011

1878-1551

Sergei Doulatov, George Q. Daley,

Epigenetics and DNA Methylation

Autophagy, a central pathway for cellular homeostasis, plays diverse roles in development, cancer, aging, and neurodegeneration. In a new report in Nature, Ho et al., 2017Ho T.T. Warr M.R. Adelman E.R. Lansinger O.M. Flach J. Verovskaya E.V. Figueroa M.E. Passegué E. Nature. 2017; 543: 205-210Crossref PubMed Scopus (493) Google Scholar show that autophagy is essential for maintaining the replicative quiescence of hematopoietic stem cells throughout life by limiting the number of active mitochondria. Autophagy, a central pathway for cellular homeostasis, plays diverse roles in development, cancer, aging, and neurodegeneration. In a new report in Nature, Ho et al., 2017Ho T.T. Warr M.R. Adelman E.R. Lansinger O.M. Flach J. Verovskaya E.V. Figueroa M.E. Passegué E. Nature. 2017; 543: 205-210Crossref PubMed Scopus (493) Google Scholar show that autophagy is essential for maintaining the replicative quiescence of hematopoietic stem cells throughout life by limiting the number of active mitochondria. Macroautophagy, or simply autophagy, has been classically viewed as a catabolic pathway induced upon starvation and metabolic stress. However, it is now appreciated that autophagy, or selective autophagy, is carried out by most cells and is crucial for the recycling of organelles and large protein complexes (Green and Levine, 2014Green D.R. Levine B. Cell. 2014; 157: 65-75Abstract Full Text Full Text PDF PubMed Scopus (520) Google Scholar). This continuous recycling is critical for maintenance of cellular homeostasis, and its role in regeneration and stem cell biology has recently come to the fore (Warr et al., 2013Warr M.R. Binnewies M. Flach J. Reynaud D. Garg T. Malhotra R. Debnath J. Passegué E. Nature. 2013; 494: 323-327Crossref PubMed Scopus (428) Google Scholar). Now, new work from the Passegué lab published in Nature shows that hematopoietic stem cells (HSCs), which sustain blood production, rely on autophagy to maintain a quiescent metabolic state throughout life (Ho et al., 2017Ho T.T. Warr M.R. Adelman E.R. Lansinger O.M. Flach J. Verovskaya E.V. Figueroa M.E. Passegué E. Nature. 2017; 543: 205-210Crossref PubMed Scopus (493) Google Scholar). These findings have important implications for therapeutic interventions to stave off aging-related declines in stem cell function. In autophagy, diverse cellular cargo are targeted into a conserved degradative pathway, which involves the formation of nascent autophagosomal membrane, lipidation of LC3 by a catalytic cascade that requires ATG12-ATG5, and targeting of the lipidated LC3 (also known as LC3-II) to the nascent membrane (Ichimura et al., 2000Ichimura Y. Kirisako T. Takao T. Satomi Y. Shimonishi Y. Ishihara N. Mizushima N. Tanida I. Kominami E. Ohsumi M. et al.Nature. 2000; 408: 488-492Crossref PubMed Scopus (1502) Google Scholar). Mature autophagosomes then fuse with lysosomes to degrade their cargo. The delivery of cargo to these vesicles is tightly regulated and mediated by chaperones. Mitochondrial autophagy (also known as mitophagy) is initiated by the loss of membrane potential in damaged mitochondria, which activates PINK1 kinase, leading to recruitment of the E3 ligase Parkin and polyubiquitination of mitochondrial proteins; these are recognized by p62, a chaperone that delivers them to autophagosomes via an LC3-interaction domain (Stolz and Dikic, 2014Stolz A. Dikic I. Mol. Cell. 2014; 56: 341-342Abstract Full Text Full Text PDF PubMed Scopus (12) Google Scholar). Failure or decline of autophagy is implicated in aging and neurodegenerative disease. Mutations in Parkin are found in familial Parkinson's disease, while p62 and other autophagy genes are often mutated in amyotrophic lateral sclerosis (Menzies et al., 2015Menzies F.M. Fleming A. Rubinsztein D.C. Nat. Rev. Neurosci. 2015; 16: 345-357Crossref PubMed Scopus (551) Google Scholar). While it has long been recognized that autophagy plays a critical role in HSCs, the long-lived quiescent cells that give rise to all blood lineages, the precise mechanisms by which autophagy influences HSCs in homeostasis and disease are just beginning to be elucidated. Deletion of core autophagy protein Atg7 compromises HSCs (Mortensen et al., 2011Mortensen M. Soilleux E.J. Djordjevic G. Tripp R. Lutteropp M. Sadighi-Akha E. Stranks A.J. Glanville J. Knight S. Jacobsen S.E. et al.J. Exp. Med. 2011; 208: 455-467Crossref PubMed Scopus (454) Google Scholar), as well as erythroid cells that require an alternative mitophagy pathway mediated by NIX (BNIP3L) to degrade mitochondria prior to enucleation (Sandoval et al., 2008Sandoval H. Thiagarajan P. Dasgupta S.K. Schumacher A. Prchal J.T. Chen M. Wang J. Nature. 2008; 454: 232-235Crossref PubMed Scopus (869) Google Scholar). HSCs can quickly turn on autophagy to cope with cellular stresses, orchestrated by FOXO3A, or increased metabolic load with the induction of Parkin-dependent mitophagy (Ito et al., 2016Ito K. Turcotte R. Cui J. Zimmerman S.E. Pinho S. Mizoguchi T. Arai F. Runnels J.M. Alt C. Teruya-Feldstein J. et al.Science. 2016; 354: 1156-1160Crossref PubMed Scopus (186) Google Scholar, Warr et al., 2013Warr M.R. Binnewies M. Flach J. Reynaud D. Garg T. Malhotra R. Debnath J. Passegué E. Nature. 2013; 494: 323-327Crossref PubMed Scopus (428) Google Scholar). However, it has remained unclear whether HSCs rely on autophagy during normal physiological processes, especially during aging. And if so, which "flavor" of autophagy is the most essential? Now, new work from the Passegué lab shows that autophagy is essential for maintaining replicative quiescence in HSCs by limiting the number of active mitochondria (Ho et al., 2017Ho T.T. Warr M.R. Adelman E.R. Lansinger O.M. Flach J. Verovskaya E.V. Figueroa M.E. Passegué E. Nature. 2017; 543: 205-210Crossref PubMed Scopus (493) Google Scholar). In this study, the authors used knockout mice for Atg12 (Atg12KO), which abolishes LC3 conjugation. These mice had higher myeloid-to-lymphoid ratio, as previously observed in other autophagy knockouts. Aging HSCs also show a myeloid bias, leading to reduced immunity with age, as well as reduced capacity for self-renewal. As the ability for regeneration decreases, the number of HSCs actually increases in the older marrow. Although the number of HSCs was unchanged in Atg12KO mice, they had reduced reconstitution potential and myeloid bias after transplantation, mimicking aging-related features. In addition, when Atg12 was knocked out after transplant, mice showed a progressive decline in donor chimerism. These experiments suggest that autophagy is required to maintain the quiescence and lineage balance of HSCs during aging. However, the authors were unable to directly compare HSCs from young and old Atg12KO mice, as the latter developed hepatomegaly due to off-target deletion effects. To uncover which flavor of autophagy was responsible for the observed phenotypes, the authors noted that Atg12KO HSCs had more active, undamaged mitochondria. This was in striking distinction with Park2KO HSCs, which accumulated more damaged dysfunctional mitochondria. Consistent with these results, Vannini et al. recently reported that HSCs have very low mitochondrial activity. Strikingly, chemical uncoupling of oxidative phosphorylation (OXPHOS) promoted mitophagy and improved maintenance of transplantable HSCs in culture (Vannini et al., 2016Vannini N. Girotra M. Naveiras O. Nikitin G. Campos V. Giger S. Roch A. Auwerx J. Lutolf M.P. Nat. Commun. 2016; 7: 13125Crossref PubMed Scopus (151) Google Scholar), indicating that pathways other than PINK-Parkin could maintain autophagic turnover of healthy active mitochondria. Atg12KO HSCs also displayed a more active metabolic state with higher levels of OXPHOS and energetic intermediates. HSCs from old mice displayed similar features of metabolic activation but had reduced levels of active mitochondria, potentially indicating accumulation of damaged mitochondria (Figure 1). These observations suggest that Parkin-dependent and -independent mechanisms may control mitochondrial accumulation in aging HSCs. Increased autophagy may help mitigate aging-related declines in cellular function. Using the LC3-GFP reporter mice to detect autophagy levels, the authors report that some old HSCs turn on autophagy, whereas others do not. These "autophagy-high" HSCs sustain higher levels of repopulation and have reduced myeloid bias. This paints a more complex picture of the heterogeneity in the stem cell compartment. Building on these results, the dynamic status of autophagy in old HSCs has to be further clarified. Because only a fraction of cytosolic LC3 is associated with autophagosomes, the level of LC3 does not provide a dynamic measurement of flux through the pathway, which requires assessment of the lipidated LC3-II isoform. Most importantly, these findings raise the possibility that enhancement of autophagy, which has already shown benefit in the context of certain blood disorders (Doulatov et al., 2017Doulatov S. Vo L.T. Macari E.R. Wahlster L. Kinney M.A. Taylor A.M. Barragan J. Gupta M. McGrath K. Lee H.Y. et al.Sci. Transl. Med. 2017; 9: 9Crossref Scopus (71) Google Scholar), may be a potential therapeutic strategy to stave off age-related decline in HSC function. The report by Ho et al., 2017Ho T.T. Warr M.R. Adelman E.R. Lansinger O.M. Flach J. Verovskaya E.V. Figueroa M.E. Passegué E. Nature. 2017; 543: 205-210Crossref PubMed Scopus (493) Google Scholar adds to the growing body of evidence implicating autophagy as a key pathway in blood system homeostasis and raises important questions for further study. Is turnover of other macromolecular components required for HSC maintenance? What are the Parkin-independent pathways for turnover of active mitochondria? And what are the relative contributions of these pathways to HSC homeostasis and aging? Lastly, the role of autophagy in hematopoietic progenitors and lineage commitment remains to be clarified. Future studies will probe the role of autophagy and related metabolic pathways in health, disease, and aging of the blood system as a whole.